1. Field of the Invention
The invention relates to tubular solid oxide fuel cells (TSOFC), and more particularly to an internal current collector comprising a tubular coil spring disposed concentrically within a TSOFC electrode, and held in firm tangential contact with the internal wall of the electrode due to the springback force of the tubular coil spring.
2. Description of the Prior Art
Fuel cells generate power by extracting the chemical energy of natural gas or other hydrogen-containing fuel without combustion. Advantages include high efficiency and very low release of polluting gases (e.g., NOx) into the atmosphere. The solid oxide fuel cell (SOFC) offers advantages of high efficiency, low materials cost, minimal maintenance, and direct utilization of various hydrocarbon fuels without external reforming. SOFCs operating with natural gas as a fuel at 1000° C. can achieve power generation efficiencies in the range of 40 to 45 percent (simple cycle). Hybrid systems combining SOFCs and gas turbines can achieve efficiencies up to 70 percent. Scientists at the United States Department of Energy's National Energy Technology Laboratory (NETL) recently developed a concept for increasing efficiencies of SOFCs to greater than 70% in a hybrid system. Their approach, called the Ultra Fuel Cell, is based on staged SOFCs where spent (but not completely combusted) fuel exiting a first SOFC stack operating at low temperature (500°-700° C.) is fed into a second SOFC stack operating at a higher temperature (>800° C.). Materials technology for high-temperature (800°-1000° C.) stacks has been developed, and could soon be commercialized for large-scale (up to 10 MW) power generation.
The ceramic materials used in high-temperature SOFCs based on current planar and tubular designs are essentially defined. Yttria-stabilized-cubic-zirconia (YSCZ) is the most widely used ceramic electrolyte membrane material because it has a high ionic conductivity and is stable in both oxidizing and reducing environments. The oxygen ionic conductivity is independent of oxygen partial pressure over a wide range of temperatures. Under these conditions, the transference number for ionic conductivity is close to unity. The use of fully stabilized YSCZ avoids problems of phase transformation associated with partially stabilized materials during cell operation. The anode material is a porous Ni—YSCZ cermet, and the cathode material is a porous (La,Sr)MnO3 (LSM) ceramic. Present designs involve the use of relatively thick YSCZ membranes, which require high operating temperatures (900°-1000° C.) to achieve optimum performance.
Approaches to reduce SOFC operating temperatures all begin with reduction of the electrolyte resistance, which can be achieved by: (1) using thin film YSCZ electrolyte membranes, (2) developing nanocrystalline materials, or (3) replacing YSCZ with a higher conductivity ceramic electrolyte material. More substantial reductions in operating temperature can be achieved by combining these approaches.
Reducing the electrolyte resistance alone, however, is not sufficient. The anode and cathode materials must be re-engineered to provide the required electrochemical performance at low temperatures. Mutual compatibility of the new materials must be established, and electrochemical cell designs and stack configurations incorporating the new materials must be developed. There has been considerable published research focused on individual components (electrolytes, anode, and cathode), as well as some preliminary efforts aimed at development of low-temperature SOFCs by co-sintering.
Current collectors used in the stacks of planar solid oxide fuel cells comprise metallic screens or some type of Ni or stainless steel plate. The planar current collectors connect the cells in series, or are used at the end of the stacks to take out the generated current. The present invention does not apply to solid oxide fuel cells having a planar current collector geometry. Rather, it relates to SOFCs of tubular construction, and more particularly to an internal current collector for use in tubular SOFC electrodes.
Tubular fuel cells manufactured by Acumentrics Corporation employ a nickel wire current collector that has been spiraled around a needle former to produce a tight coil. They then feed the coil into the tubular electrode by jamming it inside to produce a good electrical contact (Ref. 1).
In another tubular fuel cell design, Acumentrics wraps a wire coil around the inside of the electrode where it electrically contacts the electrode (Ref. 2). The same reference suggests that the wire coil can be replaced by a current-collecting mesh pushed into the tubular electrode.
In an electricity-generating fuel cell from Celltech Power, a coil spring is used to urge a flat electrical contact against a flat anode surface (Ref. 3)